Build CKEditor from sources

I am trying to build CKEditor from the sources.

I've cloned the GIT reporitory and launched the build.sh
Here is the stacktrace I get :

./dev/builder/build.sh 
CKBuilder - Builds a release version of ckeditor-dev.

Downloading CKBuilder...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 6537k  100 6537k    0     0   169k      0  0:00:38  0:00:38 --:--:--  185k

Starting CKBuilder...
Exception in thread "main" org.mozilla.javascript.EvaluatorException: Couldn't read source file "lib/json/json2.js": lib/json/json2.js (No such file or directory). (/home/wiktor/CKBuilder/src/ckbuilder.js#72)
	at org.mozilla.javascript.DefaultErrorReporter.runtimeError(DefaultErrorReporter.java:77)
	at org.mozilla.javascript.Context.reportRuntimeError(Context.java:913)
	at org.mozilla.javascript.Context.reportRuntimeError(Context.java:969)
	at org.mozilla.javascript.tools.shell.Global.load(Global.java:251)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:601)
	at org.mozilla.javascript.MemberBox.invoke(MemberBox.java:126)
	at org.mozilla.javascript.FunctionObject.call(FunctionObject.java:386)
	at org.mozilla.javascript.optimizer.OptRuntime.callName(OptRuntime.java:63)
	at ckbuilder.ckbuilder._c_anonymous_3(/home/wiktor/CKBuilder/src/ckbuilder.js:72)
	at ckbuilder.ckbuilder.call(/home/wiktor/CKBuilder/src/ckbuilder.js)
	at org.mozilla.javascript.optimizer.OptRuntime.call1(OptRuntime.java:32)
	at ckbuilder.ckbuilder._c_script_0(/home/wiktor/CKBuilder/src/ckbuilder.js:93)
	at ckbuilder.ckbuilder.call(/home/wiktor/CKBuilder/src/ckbuilder.js)
	at org.mozilla.javascript.ContextFactory.doTopCall(ContextFactory.java:394)
	at org.mozilla.javascript.ScriptRuntime.doTopCall(ScriptRuntime.java:3091)
	at ckbuilder.ckbuilder.call(/home/wiktor/CKBuilder/src/ckbuilder.js)
	at ckbuilder.ckbuilder.exec(/home/wiktor/CKBuilder/src/ckbuilder.js)
	at org.mozilla.javascript.optimizer.OptRuntime$1.run(OptRuntime.java:218)
	at org.mozilla.javascript.Context.call(Context.java:489)
	at org.mozilla.javascript.ContextFactory.call(ContextFactory.java:504)
	at org.mozilla.javascript.optimizer.OptRuntime.main(OptRuntime.java:206)
	at ckbuilder.ckbuilder.main(/home/wiktor/CKBuilder/src/ckbuilder.js)

Any idea ?